Can a pocket laser damage the eye?

December 28, 1998

This answer comes from Douglas A. Johnson, a senior health physicist and laser safety
officer for Texas A&M University. He is also adjunct lecturer in
the nuclear engineering department. Doug is a member of the American
National Standards Institute (ANSI) for laser safety standards. (The
ANSI Z136 series is recognized by OSHA, and is the authoritative laser
safety document in the United States.)

Image: CNN Interactive

LASER POINTER shines into the camera lens.

Eye damage from a pocket laser is unlikely, but could be possible
under certain conditions. Red laser pointers that are "properly
labeled" in the 3-5 mW range have not caused eye damage -- no retinal
damage has been reported -- but there are very real concerns. One is
pointers not manufactured to federal specifications. There are reports
that green lasers, improperly imported to the U.S., far exceed safety
limits.

The Food and Drug Administration's Center for Devices and Radiological Health (FDA) is responsible for
light products, including lasers. The FDA regulates the devices and
how they are classified and labeled. A class 2 is "safer" than a
class 3. Many laser pointers are in the range of 1 to 5 milliwatts (mW), a
subclass of 3 called 3A. A close reading of exposure limits indicate
that a 5 mW laser could cause eye damage.

Why even worry about 5 mW (5 thousandths of a watt), which is less than one percent of
one percent of the power of a 60 Watt incandescent bulb? First, the numbers are
used differently. Light bulb wattage measures the power it uses. It
only converts about 10 percent of that electrical power into light. In a
laser, the power is a measure of the light output.

Second, the light bulb gives light in all directions so you only see a
small part of the whole. As you move away from the bulb, you see a
quarter of the light every time the distance is doubled. A laser
gives light in one small beam. If it gets
into the eye, you receive all the laser's energy, not just a
fraction.

Third, a light bulb gives off light at many different wavelengths
(different photon energies). A laser is a pure tone, only one
wavelength. The coherent light will be more damaging.

The common red laser pointer is a diode laser, really just a special
type of transistor, or diode. Because of the unique features of laser
light, it is magnified by 100,000 times as it passes through the eye.
The light passes to the back part of the eye, the retina, which is
where we perceive vision.
The eye actually sees a small part of the electromagnetic spectrum
that runs from short cosmic ray energies to long radiowaves. We see
only from violet to red. Infrared (IR) and ultraviolet (UV) are just
outside our ability to see. The eye is most sensitive to yellow-green
light (550 nm). At the same power, 670 nm red light is only 3 percent
as bright.

So for a pointer to be useful, the power must be high enough that the
government classifies it as a 3A and requires a DANGER label.

When determining safety limits for the laser pointer or in other
areas, a value must be chosen. Above a certain number is illegal or
dangerous, below is OK. In real life many factors contribute to
something becoming harmful. Look at traffic laws. Seventy miles per hour may be
legal while 71 earns you a ticket, yet it is not really more dangerous.
But 100 mph is much riskier, and 50 mph may be dangerous if the road
is covered with ice. So with laser pointers, different conditions
determine when retinal damage will actually occur.

In FDA-regulated pointers, the laser power limit is set at one-tenth
the actual threshold of damage. If a person sees a bright light, they
will automatically blink, on the average in less than 0.2 seconds.
This is referred to as the blink reflex, and it is considered when the
limit is assigned for how much power will cause an eye injury. By the
way, you shouldn't force a stare at a laser, just like you shouldn't
stare at the sun or any bright light source.

Possible more potentially damaging -- although not to the eye -- is
that a regular pointer laser can overwhelm the eye with light,
typically called flash blindness. If a person is walking a rocky
path, operating machinery, a vehicle or aircraft, this temporary
loss of vision could cause injury or disaster. At night, when the
pupil is most open, the effects would bemagnified.

Some basic rules with lasers: Never direct a beam onto another person, especially their face. Do
not shine it onto a mirror or mirror-like surface. Do not look at the
beam through binoculars or a microscope.

One last thing -- some government entities have banned or restricted
laser pointers. Some states and some cities have or have proposed age
limits on the purchase or use of pointers. The United Kingdom bans
the use of class 3A pointers. Laser pointers are high-tech tools, not
toys.